Method of excavating



Oct 8, 1963 F. BECKENBAUER ETAL v METHOD QF ExcAvATING :Fiied April 20,1959 In venors United States Patent O 3,106,068 METHD F EXCAVATRNG FranzBeckenbauer and Ferdinand ulehla, Sulzbach- Rosenberg Hutte, Germany,assignors to Eisenwerk- Gesellschaft Maximiiianshutte A.G.,Sulzbach-Rosenberg Hutte, Germany Filed Apr. 20, 1959, Ser. No. 807,421Claims. (Cl. 61-4l) When excavating foundations or sinking shafts undercompressed air physiological considerations limit the pressures that canbe employed to 3 atmospheres gauge, which in exceptional cases may beraisedto 3.5 atmospheres gauge.

Consequently such work cannot be performed beyond a depth of 30 and inspecial circumstances beyond a maximum of 35 metres below open or groundwater level.

When using compressed air in permeable ground in which the ground waterlevel is situated at comparatively great depth the elevated pressure inthe working chamber raises the original undisturbed ground water levelin the surroundings, that is to say there is a build up in hydrostaticpressure in the neighbourhood of the caisson or shaft.

In an illustrative example which occurred in the St. Anna pit IatSulzbach-Rosenberg, this pressure build-up, ywhich is the greater thehigher the pressure inside the working chamber, amounted to as much as1.3 atmospheres gauge (the ground water level being thereby raised froma depth of 43 metres to 30 metres under the surface).

In the cited case it was therefore impossible without special approvalto continue work below normal ground water level beyond a depth of 17metres which represented the available pressure rise of 1.7 ats. gaugewhich remained after deducting the above 1.3 ats. gauge from the maximumpermissible 3 ats. gauge. The methods which were hitherto available ofpenetrating beyond 35 metres below ground water level under compressedyair consisted in lowering the ground water level by sinking springWells in the wider environment of the caisson or shaft. However,lowering the ground water level in this Way is an extremely expensiveprocedure and its success depends largely upon the grain structure andhomogeneity or inhomogeneity of the ground or rock that is thus to bedrained.

However, in the cited case at the St. Anna pit, the invention which willbe hereafter described permitted penetration to a depth of 60 metresbeyond ground water level yunder moderate pressures between 2.2 and 2.5ats. gauge. Moreover, working conditions remained entirely unchangedwhilst sinking the final 16 metres of shaft, work being carried out atthe same unvarying pressure o-f 2.2 to 2.5 ats. gauge without greaterdiiculties being experienced `at increasing depth by break-in of wateror in irruption of sand and mud. After having penetrated to a depth ofabout 60i metres below ground water level, grown rock was won, so thatthe application of compressed air in the further continuation of workceased to be necessary. Nevertheless, the invention which will bedescribed would have readily permitted work under the same unchangedconditions to be continued without any trouble. In the worst possibleground conditions the proposed method has in practice proved fullysuccessful.

A homogeneous permeable rock is even better suitable for applying themethod.

The method consists in reducing the hydrostatic pressure in the rocksurrounding the caisson or shaft by withdrawing water from the rockthrough the working chamber and discharging it into the free outsideatmosphere.

FIGURE 1 is a diagrammatic section of the invention in use;

FIGURES 2 and 3 illustrate on the left the condition prevailing withoutuse of the invention, and on the right the effect of the use of theinvention; and

'FIGURE 4 shows a drain pipe suitable for use in the invention.

In practice the method can be performed by flushing drainage pipes intothe door as well as possibly into the surrounding rock through thejoints or lining of the working chamber and connecting the same insidethe pressurised chamber by means of flexible'rein-forced tubing with aclosed pipe system which communicates with the free outside atmosphere.This simple system of piping directly discharges the water which thepressurised air forces from the ground into the drainage pipes into theoutside atmosphere. lf for reasons connected with the working procedurethe height from the bottom of the shaft to beyond the roof of thecaisson (i.e. to the free outer atmosphere) should exceed 6` metres,then yan air valve may conveniently be incorporated in the riser pipe ofthe system to act in the manner of an ejector which draws the water outof the drainage pipes and discharges it above the roof of the caisson.

During the sinking of `a shaft fat Auerbach it was observed thatalthough water entered the shaft at various points up to metres abovethe door of the shaft, indicating that the water level reached thisheight (cavities behind the wall of the shaft ywould have interconnectedany theoretically existing water tables), the pressure of the waterpenetrating the floor of the shaft did not exceed 0.6 ats. gauge. Thisfact was frequently noticed when rocks collapsed or water broke inthrough the door.

In the application of the herein described method to the work carriedout at they St. Anna pit the `following observation was repeatedly made.

During each sinking operation in the last 16 metrespenetration was atthe rate of 1.4 metres each time-flowing water brought up material fromthe centre of soft zones in the door. Drainage pipes were then flusheddown into these soft zones and connected by flexible hose to a system ofpipes leading to the free outside atmosphere. Within a -very short timethe iioor was found to consolidate and the water ceased to well up. Notmore than four such drainage pipes were thus hushed into an area (about16 sq. metres) in the shaft door. Whereas welling up of the shaft oorand irruptions of mud had repeatedly occurred at much lesser depths whenthe described method had not been employed, work proceeded according toplan and without difficulties from the moment the novel procedure hadbeen put into operation. The drainage pipes had :a diameter of 11/2lwith a filter end 1.60 rn. in length provided with 'a woven mesh No. 12surmounted by a 11/2 solid pipe section which was likewise 1.60 metreslong. When such a drainage pipe which thus had an overall length of 3.20metres had been flushed down into the ground it still remained effective`when the shaft floor had been sunk a further 11.40 metres because thefiltering surface was then still buried `another 20 cms. below the levelof the freshly sun-k floor. Before a further sinking operation beganfresh drainage pipes were washed into the ground. As soon as these werein position the old ones were disconnected and drawn.

The greater efficacy of this measure in more permeable, this is to saymore sandy rock, is due to the fact that in rock of this type theembedded drainage pipes create a much larger drainage basin, i.e. abasin of much greater volume, within the rock, Iand because a muchgreater volume of water is removed from the rnore permeable strata (cf.FIGS. 3 and 4). It may be observed that the reduction in hydrostaticpressure around the caisson or shaft in rock or soils having K-values inthe order of 10*5 cm./sec. (i.e. strata of relatively low permeability)is not accompanied by a lowering of the ground water level. By :allowingthe water to liow out of the rock via the drainage pipes through theclosed pipe system' into the free outer atmosphere the potential energyof hydrostatic pressure is converted into the kinetic energy of theflowing water. In coarse sands, gravels, and loose rock, the desiredetlect is likewise yachieved by a reduction in the `ground water level.

The above described method, on the one hand, permits the working chamberto be sun'k to a much lgreater depth below .ground water level whistmaintaining the usual pressure than was hitherto possible and, on theother hand, the depths of penetration that could in the past be achievedcan now be reached with only low pressures in the working chamber. Theselower pressures are less likely to have adverse physiological effectsand permit longer working times. Moreover, in cases in `which thesinking of caissons causes difficulties due to the ibuoyancy of the airi.e. owing to the high pressures in the working chamber, and highball-ast loads are needed in sinking, Ithis work can now be greatlyfacilitated by the reduction in the necessary pressure.

An object of the invention lies in the attainment of a lowering of thehydrostatic pressure at the point of operation without drainage of thesoil except at the point `of operation, i.e. without lowering of theground water table or the water level surrounding the caisson upwardlyof the leading edge of the caisson.

The accompanying drawings illustrate the method proposed by theinvention.

FIG. 1 shows the shaft wall at a, the ceiling of the pressurised chamberat b; the closed pipe system is indicated by c. A drainage pipe dflushed into the floor of the pressurised -chamber or pneumatic caissoncomprises an upper solid pipe section e with a flexible tube fconnecting it with the pipe system a. Additional drainage tubes g havebeen flushed into the ground behind the wall. An

,air injector h is incorporated in the riser pipe of the system whichalso includes shut-off valves at i.

For a clear understanding it is necessary only to consider that the mostimportant volume to lbe considered is the interior of the caisson. Thisvolume must ybe kept at a pressure that can be sustained by the workmenwho dig out the soil. Consider applicants drainage connection and piped, e in FIGURES 2 and 3. It will be seen that the water in the soil inadvance of caisson a will flow into well point or drainage pipe d andthe water pressure in the soil surrounding the solid pipe section e willbe reduced. There is, of course, a pressure in the caisson that is abovethe atmospheric pressure to push water above well point d downwardly,and a pressure in pipe e and f equal to not more than the static head ofwater from the caisson to the level `of the point at which it can bedisposed of by co-nvent-ional pumps. The pressure in pipes e and f maybe considerably lower, Iand preferably is lower, since the more rapidremoval of -wa-ter will mean that the pressure within the caisson can bereduced.

As seen in FIGURE 1 -at g, g, screened well points n I may be extendedfrom the sides of the shaft wall rz as required to reduce the ow ofwater downwardly adjacent the wall where the earth has been disturbed,these drainage tubes or well points are not for the purpose of reducingthe surrounding water table generally. Points g `will not lbe necessaryin all cases but only where the downflow between the wall and thesurrounding earth is excessive. The free water lbetween the earth landthe wall is then removed but no -attempt is made, or is necessary, to remove Water from the earth far beyond the space between the undisturbedearth and the wall. It will lbe seen, Ithen, that the hydrostatic headof the water surrounding the lower end of the caisson is reduced withoutparticularly iniluencing the water pressure within the undisturbed earthsurrounding the caisson above the level of the caisson end.

The structure of the drainage pipe shown lin FIGURE 4 may be anyconvention-al well point as used for the `so-called dnive wells in whichsuch a point is mounted on the end of Va pipe and ldriven into theearth. Such well points are common articles of commerce and theirstructure is adequately described by the term well point.

What we claim is:

1. A method of driving shafts or the like using a pneumatic caissonincluding the steps of projecting at least one well point into thepermeable material in the direction in which the shaft is being driven,connecting said well point to -a piping system discharging beyond saidcaisson at atmospheric pressure, and subjecting the well `point to avacuum to draw water in the permeable material in the direction awayfrom the caisson, where-by the permeable material `at the caisson isdewatered, and performing an excavating operation to lower the workingsurface in advance of the caisson.

2. The method of claim 1 in which well points are also projectedsideways through the caisson wall into the surrounding permeablemateri-al.

3. The method of claim 1, in which an ejector device is inserted in thepiping system `and w-ater owing into the well points is lifted to aspace beyond the caisson by said ejector.

4. The method of claim 1 in which said well point is driven into thepermeable material in the direction in which the shaft is being driven adistance such that the water is removed from the permeable material atleast as far beyond the working `surface as the depth of material thatis to be removed in one excavating operation.

5. The method lof claim 4, in which upon completion of one excavatingoperation at least one additional well point is driven in advance of thefirst said well point and the rst said well point is removed.

References Cited in the le of this patent UNITED STATES PATENTS 962,612Batten June 28, 1910 989,110 Billings Apr. 1l, 1911 1,010,642 KnorreDec. 5, 1911 2,126,575 Ranney Aug. 9, 1938 FOREIGN PATENTS 11,902 GreatBritain July 3, 1902 388,367 Germany June 17, 1924 264,683 Italy May 7,1929 504,427 Germany Aug. 4, 1930 698,315 Germany Nov. 7, 1940 733,806Germany Apr. 2, 1943

1. A METHOD OF DRIVING SHAFTS OR THE LIKE USING A PNEUMATIC CAISSONINCLUDING THE STEPS OF PROJECTING AT LEAST ONE WELL POINT INTO THEPERMEABLE MATERIAL IN THE DIRECTION IN WHICH THE SHAFT IS BEING DRIVEN,CONNECTING SAID WELL POINT TO A PIPING SYSTEM DISCHARGING BEYOND SAIDCAISSON AT ATMOSPHERIC PRESSURE, AND SUBJECTING THE WELL POINT TO AVACUUM TO DRAW WATER IN THE PERMEABLE MATERIAL IN THE DIRECTION AWAYFROM THE CAISSON WHEREBY THE PERMEABLE MATERIAL AT THE CAISSON ISDEWATERED, AND PERFORMING AN EXCAVATING OPERATION TO LOWER THE WORKINGSURFACE IN ADVANCED OF THE CAISSON.